An OPO generally consists of a material that has a significant non-linear coefficient of refractive index placed in a resonant optical cavity formed by two opposing mirrors. Laser radiation from a “pump” source enters the resonant cavity and some of it is converted to coherent laser radiation at a shifted wavelength for which the cavity is optically resonant and for which phase matching conditions prevail in the OPO material. This is accompanied by another laser beam, called the “idler” having a wavelength determined by the conservation of energy between the energy of a pump photon and the sum of energies of the shifted and idler photons. Non-linear materials of Potassium Titanyl Phosphate (KTP) and Potassium Titanyl Arsenate (KTA) are established to be useful OPO materials.
Shifting of the wavelength of a laser beam can, for example, be desired to provide “eye safe” laser beams. Typically, the wavelength range between 1.5 μm and 1.8 μm is referred to as an eye safe range.
Presently, there are two alternative construction designs for OPOs available, namely a discrete construction design and a monolithic design.
It is known to use a discrete construction design where the mirrors are coatings on discrete optical glass components spatially removed from the OPO. These coatings generally have good adhesion properties on optical glass.
However, this design requires alignment conditions to be maintained between the glass components, i.e. the carriers for the mirrors, and the crystal of the OPO. This design has thus disadvantages relating to costs, size, weight and the difficulty of achieving and maintaining the alignment of the separate components over the operational range of environmental conditions. Further, since the OPO crystal and the mirrors are spatially separated, there may occur unwanted Fresnel reflections from the uncoated surfaces of the crystal, hence the alignment of the crystal has to take this into consideration.
The problems of the alignment of the crystal to the mirrors are avoided in the monolithic design, where coatings are applied directly to the faces of the crystal of the OPO that then function as the mirrors for the OPO. However, the monolithic design has some disadvantages. One of these is poor adhesion of the coatings on the crystal over the required ranges of environmental conditions and the associated inconsistency of laser damage threshold of the coatings.
It will be apparent from the above that there is a need for an alternative OPO construction, which, in at least preferred embodiments, can avoid or minimise one or more of the above disadvantages, or at least provide the public with a useful alternative.